Using Plane Mesh as LightSource/Hide in Render?
You tell Blender not to render any camera rays, meaning rays that go from the camera straight to the object. To do so you have to mix an emission shader with a transparent shader. Then you need the light path node. If a ray is a camera ray you'll want to use the transparent shader. If it isn't you'll want to use the emission shader. So you have to use the "Is Camera Ray" as the mix node's factor.

Node setup:



Example render without node setup:



Example render with node setup: